1. Field of the Invention
The present invention relates to an image processing system for detecting and correcting distortion of the image, for example, density blur of a pattern and character of a printed character or a seal. The present invention is advantageously used as an image verification system of, for example, a seal or printed character on a document, bankbook, or bank check.
2. Description of the Related Art
In daily life, printed characters and seals are used for many documents, such as certificates, registrations, and notifications.
In such cases, when, for example, a seal is used, "density blur", i.e., blurring, blotting or unevenness, or the reproduction of the seal on the document sometimes occurs due to the condition of the seal. In particular, when a registered seal, for example, is used by a customer to withdraw money from a bank account, verification of a blurred and uneven seal creates a problem for the bank. Therefore, a verification system must be used by the bank in an attempt to precisely verify the authenticity of the seal.
Conventionally, there is provided an image processing method for detecting and correcting distortion (density blur and the like) of the input image, in which distortion of the input image is detected and corrected in an input apparatus of the system having a known electrical characteristic. The input apparatus comprises a correction table (memory) for each picture element of the input image.
This correction table stores a predetermined value per each picture element for correcting the input image so to obtain a uniform characteristic of the output image. When an image is input to the input apparatus, the input image is compared with the correction table. Then, the predetermined value in the correction table is added to the corresponding picture element, to correct the input image, and a new corrected picture element is obtained from the output of the input apparatus.
This system is effective when the distortion characteristics of the input image are previously known. However, it is difficult to correct distortions, such as density blur, density slope, and nonuniform line width caused by the application of a nonuniform pressure when using a seal, because these distortions occur for a variety of reasons and the correction table of the input apparatus can not follow all the variations of such distortions.
This problem will be explained in detail hereinafter.